An industrial effluent treatment plant treats liquid effluents in order to reduce the polluting load of effluents before discarding them. This reduction is implemented by trapping mineral and organic pollutants by precipitation as insoluble compounds which, by coagulation-flocculation and decantation, form residual sludges.
French regulations impose inertization of ultimate hazardous industrial wastes before their storage in a storage center of class 1. Inertization of these wastes designates their stabilization and their solidification. Stabilization allows reduction in the solubility of the pollutants by their transformation into a form not very mobilizable by the environment. Solidification allows improvement in the physical and mechanical properties of the waste, and facilitates its handling and its storage.
Also, plants for treating radioactive liquid effluents have the role of collecting, storing and treating radioactive aqueous effluents. The treatment uses a co-precipitation method for decontaminating the effluents. Treatment by co-precipitation allows the radio-elements to be trapped by adsorption or by ion exchanges in precipitates of different natures. Decontamination of the radio-elements is carried out by using adsorbents. Some of them, such as ferrous and copper hydroxides or barium sulfate, are generated in situ, in the effluent to be decontaminated. Other ones are preformed, such as mixed nickel and potassium ferocyannide (ppFeNi) used for trapping cesium.
After precipitation and adsorption reactions, the suspension undergoes an operation for flocculation and for solid-liquid separation by decantation. After decantation, a fraction of the decontaminated supernatant is drained off to be discharged, the remainder being drained off towards the solid-liquid separation operation by filtration. Solid-liquid separation is achieved for example on a rotary filter with a filtering medium. It produces a decontaminated filtrate and <<ultimate waste>> sludges which are then conditioned.
Cementation is one of the techniques allowing inertization of wastes, notably of industrial and/or radioactive wastes appearing as sludges.
More particularly, the cementation of sludges conventionally consists in carrying out kneading (sludge+hydraulic cement) and then of casting the obtained mixture into a container. The good quality of the kneading for obtaining a homogeneous mixture at a macroscopic level as well as the possibility of casting the entirety of the coated material are inescapable conditions for using such a method.
Inertization in a cement matrix however has difficulties inherent to the physico-chemistry of the sludges to be treated. Indeed, suspensions of metal hydroxides in the case of iron for example have non-Newtonian rheological behavior characterized by flow behavior characterized by high flow threshold stress and high plastic viscosity.
Under certain conditions, suspensions of iron hydroxides have a structure of the gel type and a strong increase in the flow parameters is observed. This structure of the gel type is the limiting case of concentrated suspensions, in which all the liquid of the dispersing phase is trapped in structural units of the suspension which here are precipitate aggregates. The formation of a structure of the gel type from a suspension may be accomplished at rest in several days or much more rapidly, during the flowing of the suspension. The solid concentration, the salt content and the applied shearing to the suspension are the parameters which influence rheology in the first order. During cementation of sludges of metal hydroxides, the water contained in the sludge is bound to the solid and is not available for ensuring suitable rheology of the slurry. The rheological (flow) behavior of the mixture depends on the rheological behavior of the sludge. Cementation of this type of waste requires the use of a high water/cement (W/C) mass ratio in order to obtain the desired workability or consistency. Now, increasing the W/C ratio increases porosity, reduces mechanical strengths, which may thus no longer attain the required compression threshold (typically 8 MPa), without forgetting a significant reduction in the incorporation level.
Further, during the cementation of wastes, many chemical species contained in the sludges may precipitate and cause rapid stiffening of the mixture or flash setting in the kneader, phenomena which are also potentially redhibitory.
Therefore there exists a real need for proposing a method which is easy to apply and allowing inertization of the wastes and notably of industrial and/or radioactive wastes advantageously appearing as sludges.